19. Which of the following statements regarding hotmone and cellular signal transduction is correct?
(1) Amino acid and protein hormones usually act by binding to a receptor on the cell surface.
(2) A second messenger is a substance that binding to its receptor on the cell surface.
(3) cATP is an intracellular second messenger.
(A) (1)
(B) (2)
(C) (3)
(D) (1) and (3)
(E) (2) and (3)
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